1. Field of the Invention
The present invention relates to an optical storage device and emission control method for recording and regenerating information on and from a storage medium.
2. Description of the Related Art
The advancement of technology in the information recording field is remarkable, and research and development are energetically progressing for magneto-optical disk memories, optical disk, and optical cards in terms of high density recording and regeneration, and high-speed access. In such optical storage devices, a laser diode is often used for the light source.
FIG. 13 is a diagram depicting the configuration of a conventional optical storage device, and FIG. 14 is a diagram depicting a conventional high frequency superimposing method. As FIG. 13 shows, the light emitted from the semiconductor laser element (laser diode) 90 transmits through the optical system (beam splitter 91, rise mirror 94, etc.), is focused by the objective lens 92, and is irradiated onto the recording medium (disk) 93.
The reflected light from the recording medium 93 returns through the path in which the light came through, and a part of the light returns to the laser element 90 via the beam splitter 91. It is known that the reflected light is also received by the detector 95 via the beam splitter 91, and the regeneration signal, track error signal and focus error signal are generated. By using this track error signal and focus error signal, track servo and focus servo control are performed so that the optical beam follows up the track of the recording medium, and follows up to the focal point.
When the laser beam returns to the semiconductor laser element 90 in this way, the laser beam makes oscillation inside the semiconductor laser element 90 unstable, and fluctuates the laser output. This may cause changes in the quantity of the return light which returns to the detector 95, making the above mentioned servo control status unstable as well. At worst the track servo and focus servo tend to be OFF.
This unstable status does not always occur, but occurs in a certain status under the influence of the temperature of the laser element and the optical path length of the laser beam, etc. It is possible to decrease the return light to the laser element by adding an element to the optical system, but this addition increases the cost.
So a method called high frequency superimposing method (HFM) is adopted as a method to prevent this fluctuation of the laser beam. This method is not used an only DC laser drive current, but superimposes a high frequency signal 98, which is a several hundred MHz frequency, onto the laser drive current. By this, oscillation in the semiconductor laser element becomes multi-mode, and the influence of the return light to the laser element decreases. As a result, the unstable emission of the laser element is solved and servo is stabilized.
In such a high frequency superimposing method, pulse emission is performed according to the write data during the write operation, and a similar effect can be obtained without superimposing a high frequency, as shown in FIG. 14. Therefore generally a high frequency is superimposed during erase and read of a write command (see Japanese Patent No. 2731237, for example).
To superimpose a high frequency, however, extra current (several tens of mA) is required to oscillate high frequency, which may cause a negative influence, such as heating. So a method of increasing the superimposing amount when necessary, not applying a high superimposing amount constantly, has been proposed (e.g. Japanese Patent Laid-Open No. H5-197994).
This proposal discloses that high frequency is superimposed all the time, but the superimposing amount is increased when focus entry, or the superimposing amount is increased at track off when track control can not maintain, or the superimposing amount is increased at retry of a focusing error. In other words, the high frequency superimposing amount is increased to prevent noise at focus entry, or when track servo off, since the return light amount increases at that time.
Recently such optical disk drives are increasingly used for mobile equipment. Particularly for battery-powered equipment, a decrease of power consumption is an important issue, where power reduction in mA units is requested. For this point of view, high frequency superimposing is not preferable, and should be avoided if possible.
In the recent USB (Universal Serial Bus) interface standard, the power supply amount is controlled to be a predetermined value, and power capacity beyond necessity cannot be received. Operation within the range of the power supply amount is certainly possible, but power exceeding this may be required depending on the operation status, and in such a case the disk drive stops. In other words, there is no extra power supply, and the margin of the power supply must be increased by minimizing the power consumption.
In the above mentioned prior art, the high frequency superimposing amount is decreased during focus servo control and track servo control, but operation may become unstable even if focus servo and track servo control are ON, so the effect of high frequency superimposing is low, and power consumption loss is rather high. If the high frequency superimposing amount is increased during focus servo and track servo control, on the other hand, power consumption cannot be decreased.
Also to control the increase/decrease of the high frequency superimposing amount, the circuit configuration becomes complicated, which is not good in terms of cost.